Computer-assisted surgery (CAS) systems provide position and orientation information in different forms throughout the operative steps, to guide the surgeon in his/her decision making. CAS systems are used for instance to assist surgeons in hip replacement surgery. In hip replacement surgery, the hip joint implants being implanted must assure a desired posture to the patient. Accordingly, the position and orientation information provided to the surgeon must be precise and accurate to obtain the desired posture.
The femoral implant and the acetabular implant generally form a spherical joint, in which the center of a ball head of the femoral implant coincides with the center of an hemispherical socket of the acetabular implant, at a center of rotation of the hip joint implant. During surgery, the femur is separated from its associated pelvis for the implants to be implanted. Through the separation of the femur from the pelvis, position and orientation information is still provided from the tracking of the femur, the pelvis and the various tools being used. For instance, a rasping tool altering the intramedullary canal of the femur may be tracked such that the center of rotation of the femoral implant (i.e., the center of the ball head) may be calculated as a function of the geometry of the femoral implant and of the altered intramedullary canal.
Some types of femoral implants come separate with the ball head being fixable to the femoral implant body. The femoral implant body has a frusto-conical connector end (e.g., a Morse 12/14 taper) upon which the ball head is slid in a friction fit. In calculating the position of the center of rotation of the femoral implant, some precision is lost considering that the fit between the ball head and the frusto-conical connector end is unpredictable to some extent.
Alternatively, it may be desired to confirm the position and orientation of the femoral implant. Referring to the above-described example in which the center of rotation of the femoral implant is calculated as a function of the geometry of the femoral implant and of the altered intramedullary canal, it is possible that the femoral implant is not completely fitted as expected in the altered intramedullary canal. In such a case, a confirmation of the position and orientation of the femoral implant would be appropriate.